Early blood RNA clearance and protein fraction profiles predict treatment outcomes in cats with effusive feline infectious peritonitis

ObjectivesThe present study retrospectively examined effusive feline infectious peritonitis (FIP) cases to investigate whether baseline viral RNA loads and serum biomarkers are associated with treatment responses and to identify early prognostic indicators that will guide clinical decision-making.MethodsA total of 15 cats with effusive FIP that presented to a primary care veterinary hospital in Japan between August 2024 and August 2025 were included. The diagnosis was based on the European Advisory Board on Cat Diseases guidelines, combining clinical presentation, laboratory findings and feline coronavirus (FCoV) RNA detection by RT-qPCR. Antiviral treatment included GS-441524, remdesivir, molnupiravir or adjunctive nirmatrelvir. Cats were retrospectively classified as high-responders (HRs), low-responders (LRs) or non-responders (NRs), based on the blood FCoV <i>N</i> gene RNA load 2 weeks after treatment initiation. LR and NR cats were combined (LR/NR, n = 10) in analyses. Viral RNA loads in ascitic fluid and blood, routine haematology, acute-phase proteins and serum protein fractions were compared between groups.ResultsAt treatment initiation, the LR/NR group had significantly higher blood <i>N</i> gene RNA loads (<i>P</i> <0.01) and ascitic fluid RNA loads (<i>P</i> <0.05) than the HR group. In contrast, no intergroup differences were detected in <i>M</i> gene loads. Routine haematological markers revealed higher total protein, globulin (Glb) and lactate dehydrogenase in the LR/NR group, and no significant differences in albumin (Alb), total bilirubin or serum amyloid A. A serum protein fraction analysis showed distinct profiles: the HR group had higher albumin:globulin ratios and higher Alb, alpha (α)1-, α2- and beta-Glb fractions, while the LR/NR group had a markedly higher gamma (γ)-Glb fraction. The persistence of blood viral RNA 2 weeks after treatment initiation, together with opposing changes in the α2- and γ-Glb fractions, emerged as promising predictors of treatment outcomes.Conclusions and relevanceBaseline blood <i>N</i> gene RNA loads and serum Glb fractions have potential as early prognostic indicators of therapeutic responses in effusive FIP. Some of these results support the utility of combining viral and host biomarkers to improve outcome predictions and treatment monitoring.